About 600 Mbox here on a VM under Hyper-V.

No issue so far...

i don't know with 9500 mboxes....

We're running on a VMWare VM with about 4600 mailboxes and 117 domains.

Also no issues.

I would still recommend Bare-Metal as I/O bottlenecks can still be a big issue on VMs unless you have a RAID Array of all SSDs or a very fast SAN. That said, we are running SM for 275 domains and 1636 users on a VM under Hyper-V. Under heavy load it is not unusual for us to start pushing 350+ IOPS which causes a high Current Disk Queue Length > 10-16  and our Disk Idle Time is less than 1%. This results in periods of noticeable slowness to occur within the SM web interface and a several second delay in message deliveries, slowness with IMAP\EWS\EAS synchronization and more as the VM must queue the Read\Write operations rather than committing said operations to the disk in real time. We are probably going to have to migrate back to Bare-Metal soonerish.

Run performance monitors on your current installation first, especially at peak times, to get a reasonable expectation of what kind of I/O & IOPS you are going to need. It may exceed the performance you can reasonably expect from most VM deployments as we are experiencing.